18.38Secondary dominant scale problem

Now consider:

Cmaj7 → E7 → Am7

C major alone is no longer enough because E7 has:

G♯

So over E7, you need to acknowledge the chord.

Simplest strategy:

Don't choose a scale yet. Use E7 chord tones.

E G♯ B D

plus nearby approach notes.

This is still often better than immediately memorizing a new mode.